The use of three-terminal power semiconductor devices in rectifier or AC switching circuits is well known in the art. Three-terminal power semiconducting devices as used herein refers to the class of devices including SCRs, thyristors and bipolar transistors that are rated for conducting load currents and controllable to operate in the conducting state by application of a control signal to a third terminal of the device. While for clarity of explanation the ensuing discussion is directed in particular to SCRs, it is understood to be equally applicable to three-terminal devices in general.
With respect to rectifier circuits such as may be constructed with SCRs, the SCRs are typically connected as a three phase, full wave rectifier bridge consisting of three pairs of SCRs, in which a different pair of SCRs is connected to rectify the AC voltage of a different phase of a three phase AC source. A DC voltage is provided on the output of the rectifier circuit, the amplitude of the DC output voltage and the power delivered to an electrical load connected to the rectifier output being controlled by controlling the respective conduction angles of the SCRs. More specifically, each SCR can conduct during the half cycle of the AC source phase that causes the SCR anode to be positive with respect to its cathode. However, the SCR does not conduct unless a gate signal is applied to the third or gate terminal of the SCR. Thus, upon so applying the gate signal, the SCR provides a conductive path between its cathode and anode terminals, i.e., the SCR turns on, and remains conductive so long as the anode current through the SCR is above the holding current specification of the SCR device. The application of the gate signal causing the SCR to conduct is variously referred to in the art as turning on, firing or triggering the SCR.
Conduction angle as used herein refers to the portion, in degrees, of the 180 degree half cycle of the power source phase applied to the SCR during which the SCR conducts. Typically, the SCRs are turned on during the source voltage phase half cycle and remain conductive for the duration of the half cycle. In such a case, the conduction angle is measured relative to the next zero crossing of the AC phase voltage applied to the SCR. As a result, as the SCR is turned on later during the half cycle, the conduction angle becomes shorter. Further, as the conduction angle decreases, a lower DC voltage is generated on the rectifier circuit output. A load for such a rectifier circuit can typically comprise an inverter that is in turn connected to supply power to an AC load, e.g., an AC motor. Thus, control of the SCR conduction angles in the rectifier circuit provides means for controlling operation of the AC load.
Three-terminal power semiconductor devices, such as SCRs, are also applied to construct an AC switch that is used to regulate directly the power flow to an AC load. An AC switch is typically constructed with a different pair of SCRs connected between each phase of a three phase AC source and a corresponding phase connection of a three phase AC load, e.g., a three phase motor. As is known, with respect to each pair of SCRs, the two SCRs are connected in parallel with the anode of one device connected to the cathode of the device with which it is paired. As a result, one of the SCRs can be turned on to conduct during each half cycle of the AC source phase with which they are associated. Regulation of the power delivered to the AC load is achieved by controlling the conduction angles of the respective SCRs of the AC switch.
The control circuits known in the art, for generating gating signals to control three-terminal device operation in rectifier or AC switch circuits, must operate in phase with the AC source since the conduction angle of each SCR is, typically, measured relative to the zero crossing of the associated phase voltage of the AC source. In order to achieve such in-phase operation, an an AC reference signal is derived from at least one phase, and in some cases all three phases, of the AC source, each reference signal being used for generation of the gating signals. An exemplary SCR control circuit, for generating signals to control the SCRs of a rectifier or AC switch, is disclosed in commonly assigned U.S. Pat. No. 4,499,534, the disclosure of that patent being incorporated in its entirety herein by reference. The control circuit disclosed in that patent uses an AC reference voltage signal derived from a single phase of the three phase AC source and from that reference signal generates the gating signals applied to the SCRs connected to all three phases of the AC source.
In particular, the control circuit disclosed in the '534 patent includes three separate ramp forming circuits that are respectively controlled by signals generated from the reference voltage, the three ramp circuits being respectively associated with the three phases of the AC power source. Each of the three ramp signals generated by the ramp circuits is applied to a separate comparator circuit concurrent with a DC control voltage proportional to the power to be transferred to the load. Each ramp circuit generates a ramp shaped waveform that declines from an initial voltage value determined by a reference voltage, the rate of decline being determined by the integrator circuit of the ramp circuit. If the rates of decline of the three ramp circuits are not identical to one another, the durations of the respective gating signals generated therefrom will vary. Such differences in gating signal durations, i.e., differences in the conduction angles of the respective SCRs, will result in unbalanced load current, such unbalanced currents being undesirable due to their adverse effect on operation of the load.
In addition to the above disadvantage, the control circuit disclosed in the U.S. Pat. No. 4,499,534 requires ramp reset circuitry comprising a plurality of digital counters to generate signals for controlling operation of the three separate ramp circuits so that the generated ramps have a phase relationship corresponding to the three phases of the AC source. Such ramp reset circuitry, in addition to the three ramp circuits and three corresponding comparator circuits, contribute to the overall parts count of the control circuit, it being desirable from an economic standpoint to minimize the circuit parts count. Further, the overall circuit reliability can decline with increasing parts count. A further disadvantage of employing that control circuit is the expenditure of labor required to calibrate the ramp circuits. An additional disadvantage derives from the possibility that, over time, the operation of the separate ramp and comparator circuits may become unbalanced due to drift in device characteristics.